Interior appears larger than exterior

I am thinking you could do this by having two separate areas. One area models the exterior, another area models the interior. You could have a camera following each so they are in sync, and let the second camera render onto only where a mask has rendered, which should be your windows and doorways. Once your character walks into, or near, or through the doorway, teleport the character into the building. It may be hard to get it to look seamless, but with some practice and with some special shaders to address cutting out the windows I think it should be doable. You may need to also animate the fov and projection settings of the camera as you go through, although I am only speculating as I haven't really done this before, but played with the idea of blending between two completely different areas with nice looking shader effects.

An example for rendering masks, you could render alpha or depth or some other kind of texture mask to produce the cutout, with special geometry placed along your windows. The second camera should only render onto those masked areas while looking from outside.

It's not a very detailed recepie I give you, but to go through this in detail would take a fair amount of time, at least for me. I hope you will be able to proceed forward on your own and that someone else comes by and leaves their thoughts on the problem to get some diversity. It may be easier than this, I just don't know.